Electrolytes with strong oxidizing additives for lithium/sulfur batteries

ABSTRACT

Disclosed are oxidizer-treated lithium electrodes, battery cells containing such oxidizer-treated lithium electrodes, battery cell electrolytes containing oxidizing additives, and methods of treating lithium electrodes with oxidizing agents and battery cells containing such oxidizer-treated lithium electrodes. Battery cells containing SO 2  as an electrolyte additive in accordance with the present invention exhibit higher discharge capacities after cell storage over cells not containing SO 2 . Pre-treating the lithium electrode with SO 2  gas prior to battery assembly prevented cell polarization. Moreover, the SO 2  treatment does not negatively impact sulfur utilization and improves the lithium&#39;s electrochemical function as the negative electrode in the battery cell.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to lithium-sulfur batteries, andin particular to battery electrolytes having additives of oxidizingagents.

[0002] The rapid proliferation of portable electronic devices in theinternational marketplace has led to a corresponding increase in thedemand for advanced secondary batteries. The miniaturization of suchdevices as, for example, cellular phones, laptop computers, etc., hasnaturally fueled the desire for batteries having high specific energies.In addition, heightened awareness concerning toxic waste has motivated,in part, efforts to replace toxic cadmium electrodes in rechargeablenickel/cadmium batteries with the more benign hydrogen storageelectrodes in nickel/metal hydride cells. For the above reasons, thereis a strong market potential for environmentally benign batterytechnologies.

[0003] Secondary batteries are in widespread use in modem society,particularly in applications where large amounts of energy are notrequired. However, it is desirable to use batteries in applicationsrequiring considerable power, and much effort has been expended indeveloping batteries suitable for high specific energy, medium powerapplications, such as, for electric vehicles and load leveling. Ofcourse, such batteries are also suitable for use in lower powerapplications such as cameras or portable recording devices.

[0004] At this time, the most common secondary batteries are probablythe lead-acid batteries used in automobiles. These batteries have theadvantage of being capable of operating for many charge cycles withoutsignificant loss of performance. However, such batteries have a lowenergy to weight ratio. Similar limitations are found in most othersystems, such as Ni-Cd and nickel metal hydride systems.

[0005] Among the factors leading to the successful development of highspecific energy batteries, is the fundamental need for high cell voltageand low equivalent weight electrode materials. Electrode materials mustalso fulfill the basic electrochemical requirements of sufficientelectronic and ionic conductivity, high reversibility of theoxidation/reduction reaction, as well as excellent thermal and chemicalstability within the temperature range for a particular application.Importantly, the electrode materials must be reasonably inexpensive,widely available, non-toxic, and easy to process.

[0006] Thus, a smaller, lighter, cheaper, non-toxic battery has beensought for the next generation of batteries. The low equivalent weightof lithium renders it attractive as a battery electrode component forimproving weight ratios. Lithium provides also greater energy per volumethan do the traditional battery standards, nickel and cadmium.

[0007] The low equivalent weight and low cost of sulfur and itsnontoxicity renders it also an attractive candidate battery component.Successful lithium/organosulfur battery cells are known. (See, De Jongheet al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S.Pat. No. 5,162,175.) Recent developments in ambient-temperature sulfurelectrode technology may provide commercially viable primary andrechargeable lithium-sulfur batteries. Chu and colleagues are largelyresponsible for these developments which are described in U.S. Pat. Nos.5,582,623 and 5,523,179 (issued to Chu). The patents disclose asulfur-based positive electrode for a battery cell that has lowequivalent weight and high cell voltage and consequently a high specificenergy (greater than about 120 Wh/kg). The disclosed positive electrodeaddresses deficiencies in the prior art to provide a high capacitysulfur-based positive composite electrode suitable for use with metal(such as lithium) negative electrodes. These developments allowelectrochemical utilization of elemental sulfur at levels of 50% andhigher over multiple cycles. Because sulfur has a theoretical maximumcapacity of 1675 mAh/g (assuming all sulfur atoms in an electrode arefully reduced during discharge), the utilization of sulfur inlithium-sulfur cells as described in the above Chu patents typicallyexceeds 800 milliamp-hours per gram (mAh/g) of sulfur.

[0008] Nevertheless, the challenge of providing improved batteries, andespecially batteries having increased cycle life and long shelf-life,remains. In particular, the shelf-life of lithium-sulfur batteries islimited by the degradation of the lithium electrode surface during cellstorage and the formation of thick and resistive surface passivatingfilm comprising Li₂S. The passivating film may significantly increasethe lithium electrode polarization at the early stages of the celldischarge.

[0009] To compensate for this loss of active anode material, extralithium may be provided for the lithium electrode increasing the costand weight of the battery. The use of additional metals also increasesthe burden of disposing of the battery as additional toxic materialsmust be processed. Mossy lithium formed during cell cycling can alsopresent a fire hazard by creating fine particles of lithium metal thatcan ignite on contact with air. Accordingly, methods for the preventionof capacity loss in battery cells with sulfur-containing cathodes andthe prevention of degradation of the surface of a lithium electrode insuch cells would be desirable.

SUMMARY OF THE INVENTION

[0010] The present invention provides oxidizer-treated lithiumelectrodes, battery cells containing such oxidizer-treated lithiumelectrodes, battery cell electrolytes containing oxidizing additives,and methods of treating lithium electrodes with oxidizing agents andbattery cells containing such oxidizer-treated lithium electrodes.Battery cells containing an SO₂ oxidizing agent as an electrolyteadditive in accordance with the present invention exhibit higherdischarge capacities after cell storage over cells not containing SO₂.Pre-treating the lithium electrode with SO₂ gas prior to batteryassembly prevented cell polarization. Moreover, the SO₂ treatment doesnot negatively impact sulfur utilization and improves the lithium'selectrochemical function as the negative electrode in the battery cell.

[0011] One aspect of the invention provides a battery cell electrolyte.The battery cell electrolyte may be characterized as including: a) amain solvent of an electrolyte solvent mixture, having the chemicalformula R₁(CH₂CH₂O)_(n)R₂, where n ranges between 1 and 10, R₁ and R₂are different or identical groups selected from the group consisting ofalkyl, alkoxy, substituted alkyl, and substituted alkoxy groups and b)an oxidizing agent additive comprising no more than about 49% by weightof the electrolyte solvent mixture. The oxidizing agent additive may beat least one of sulfur dioxide, nitrous oxide, carbon dioxide, ahalogen, an interhalogen, an oxychloride and a sulfur monochloride wherethe halogen is selected from the group consisting of Cl₂, Br₂ and I₂. Inspecific embodiments, the oxychloride may be selected from the groupconsisting of SO₂Cl₂ and SOCL₂ and the interhalogen may be selected fromthe group consisting of iodine monochloride (ICl), iodine trichloride(ICl₃) and iodine monobromide I₂Br₂. Typically, the oxidizing agentadditive has a stronger oxidizing ability than elemental S.

[0012] In preferred embodiments, the electrolyte may include a dioxolaneco-solvent where the dioxolane co-solvent comprises less than about 20%by weight of the electrolyte solvent mixture and a second co-solventhaving a donor number of at least about 13. The main solvent may be fromthe glyme family, in particular 1,2-dimethoxyethane (DME). Theelectrolyte may include an electrolyte salt where the electrolyte may bein a liquid state, a gel state or a solid state.

[0013] Another aspect of the present invention provides a battery cell.The battery cell may be characterized as including: a) a negativelithium electrode b) a positive electrode comprising anelectrochemically active material and c) an electrolyte including a: i)a main solvent of an electrolyte solvent mixture, having the chemicalformula R₁(CH₂CH₂O)_(n)R₂, where n ranges between 1 and 10, R₁ and R₂are different or identical groups selected from the group consisting ofalkyl, alkoxy, substituted alkyl, and substituted alkoxy groups and ii)an oxidizing agent additive. The oxidizing agent additive may be atleast one of sulfur dioxide, nitrous oxide, carbon dioxide, halogens,interhalogens, oxychlorides and sulfur monochlorides. Theelectrochemically active material may comprise sulfur in the form of atleast one of elemental sulfur, a metal sulfide, a metal polysulfide, anorganosulfur material, and combinations thereof, wherein said metal isselected from the group consisting of alkali metals, alkaline earthmetals, and mixtures of alkali and alkaline earth metals.

[0014] In specific embodiments, the battery cell electrolyte may includedioxolane as a co-solvent, comprising no more than 20% by weight of theelectrolyte solvent mixture and a high donor number co-solvent having adonor number of at least about 13. In addition, the battery cellelectrolyte may include an electrolyte salt. The electrolyte of thebattery cell may be in a liquid state, a gel state, or a solid state.

[0015] Another aspect of the present invention provides a method ofmaking a protected lithium electrode battery cell. The method may becharacterized as including: a) treating a lithium material with anoxidizing agent to form a negative electrode having a protective film,b) forming a positive electrode comprising an electrochemically activematerial and c) combining said negative and positive electrodes with anelectrolyte following the treating of said lithium material where theoxidizing agent is at least one of sulfur dioxide, nitrous oxide, carbondioxide, halogens, interhalogens, oxychlorides and sulfur monochlorides.

[0016] Another aspect of the present invention provides a method ofmaking a protected lithium electrode battery cell. The method may becharacterized as including: a) forming a negative electrode comprising alithium material, b) forming a positive electrode comprising anelectrochemically active material and c) combining said negative andpositive electrodes with an electrolyte containing an oxidizing agentadditive wherein the oxidizing agent additive reacts with the lithiummaterial of the negative electrode to form a protective film on thenegative electrode's surface. In specific embodiments, the negativeelectrode may be a glassy coated lithium electrode where a crack in theglassy coated lithium electrode may be penetrated by the oxidizing agentadditive and the crack may be filled with a reaction product between theoxidizing agent additive and the lithium material of the glassy coatedlithium electrode. These and other features of the invention willfurther described and exemplified in the drawings and detaileddescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 depicts representative discharge characteristics for Li/Scells with and without an oxidizing agent additive.

[0018]FIG. 2 is a block diagram of lithium/liquid electrolyte/sulfurcell of this invention.

[0019]FIG. 3A is a schematic illustration of a pre-formed barrierlaminate including a bonding layer on a polymer or glass barrier, whichis in turn on a carrier.

[0020]FIG. 3B is a schematic illustration of a lithium electrode beingprepared according to an embodiment of the invention including forming alithium layer on the bonding layer of the barrier layer laminateillustrated in FIG. 3A.

[0021]FIG. 4 illustrates effect of SO₂ electrolyte additive on dischargecharacteristics of Li/S cells after storage.

[0022]FIG. 5 illustrates effect of Li electrode pretreatment with SO₂ ondischarge characteristics of Li/C cells containing dissolvedpolysulfides after cell storage.

[0023]FIG. 6 illustrates effect of combination of Li electrodepretreatment with SO₂ and SO₂ electrolyte additive on dischargecharacteristics of Li/S cells after storage.

[0024]FIG. 7 illustrates effect of Li electrode pretreatment withthionyl chloride on Li electrode impedance in electrolyte containingdissolved polysulfides during cell storage.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] Reference will now be made in detail to preferred embodiments ofthe invention. Examples of preferred embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these preferred embodiments, it will be understood thatit is not intended to limit the invention to such preferred embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order toprovide. a thorough understanding of the present invention. The presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

Introduction

[0026] The present invention provides oxidizer-treated lithiumelectrodes, battery cells containing such oxidizer-treated lithiumelectrodes, battery cell electrolytes containing an oxidizing agent, andmethods of treating lithium electrodes with an oxidizer and methods ofconstructing battery cells containing such oxidizer-treated lithiumelectrodes. Treating lithium with an oxidizing agent allows a solidpassivating film with Li-ion conductivity to form on the surface of thelithium. When used in a battery cell, the passivating film may protect aLi electrode from reactions with other active components of theelectrolyte. When the Li is not treated with an oxidizing agent, duringcell storage, the reactions with other active components in theelectrolyte may result in the formation of a thick and porous film onthe lithium surface. The thick and porous film may result in cell ratecapability loss and a significant increase in Li electrode polarizationat the early stage of the cell discharge. Pre-treatment of the Lielectrode with the oxidizing agent or in-situ treatment of the Lielectrode using an oxidizing additive in the electrolyte reduces therate capability loss and polarization effects in a battery cell ascompared to untreated Li electrodes in a similar battery cell. Inaddition, the oxidizing agent treatment does not negatively impactsulfur utilization and improves the lithium's electrochemical functionas the negative electrode in the battery cell.

[0027] For example, experiments with Lithium-sulfur cells havingS-loaded carbon cathodes and different electrolytes have shown thatafter storage for several days cell capacity significantly decreased.The capacity loss was observed for electrolyte formulations with a highsolubility or a low solubility of lithium polysulfides (PS). Withoutwishing to be bound by any particular theory, the capacity loss may becaused by the dissolution of elemental sulfur in the electrolytefollowed by reaction between sulfur and the lithium electrode surface.The product of the reaction of sulfur with the lithium surface (Li₂S)dissolves in the electrolyte to form lithium polysulfides (Li₂S_(x))Thus, the corrosion rate of the Li electrode will be high as long aselemental sulfur is present in the cell. Further, the formation of aLi₂S layer on the Li electrode surface may result in high electrodepolarization when the cell is initially discharged.

[0028] The pre-treatment or in-situ treatment of the lithium electrodewith the oxidizing agent decreases the corrosion and polarizationeffects. For example, when the lithium electrode is pre-treated with SO₂gas, it is believed that a protective layer of the Li-ion conductorLi₂S₂O₄ is formed in the surface of the electrode. The Li₂S₂O₄ layer mayprevent reaction of elemental sulfur with the surface of the electrodeand hence decrease electrode corrosion during cell storage andpolarization of the electrode during discharge.

[0029] In addition, the oxidizing agent may be advantageously used asadditive in lithium battery cell electrolytes. For example, when the SO₂is used in certain amounts as an additive in the electrolyte, preferablyno more than about 49% by weight of the electrolyte solvent mixture, SO₂can provide in situ protection of the lithium electrode withoutnegatively impacting sulfur utilization while improving cell dischargecharacteristics after storage under open circuit voltage (OCV)conditions. As a result of presence of SO₂ electrolyte additive, thehighest state of sulfur oxidation due to the oxidation of polysulfidesto sulfur is maintained.

[0030] The capacity of the battery cell may be increased employing anoxidizing agent additive in the electrolyte solution. For battery cellscontaining an oxidizing agent additive, the cathode has the value of thepotential between the potential of the cathode, including the sulfurelectrode, and the potential of the oxidizing additive which is usuallymore positive than the potential of lithium. When the cells starts todischarge, the oxidizer is reduced first which may increase the capacityof the battery. The additional delivered capacity depends on the amountof the oxidizing agent dissolved in the electrolyte. Further, oxidizingagents may alter the chemistry in a Li/S in a number of other ways whichmay also be beneficial to the battery cell performance: 1) via theformation of charge-transfer complexes between additives and organicsolvents or polysulfides in the cell, 2) via the electrocatalysis ofsulfur and polysulfides and 3) via a change in the morphology of theLi₂S precipitate due to the formation of discharge products of theoxidizing additive.

[0031]FIG. 1 are example plots of discharge curves after storage underOCV conditions, for Li/S battery cells with and without oxidizing agentadditives. The dashed line is representative of a typical dischargecurve for a Li/S battery cell with the oxidizing agent additive and thesolid line is representative of a typical discharge curve for a Li/Sbattery cell without the oxidizing agent additive. Other than theelectrolyte oxidizing agent additive, both cells utilize a similarelectrolyte formulation. Details of the electrolyte formulation aredescribed in more detail in the following section on electrolytes.

[0032] The larger capacity of the battery cell with the oxidizing agentadditive during initial discharge of the battery is due to the reductionof the oxidizing agent. When the oxidizing agent additive is reducedcompletely, the battery cells with and without the oxidizing agentadditive exhibit a similar capacity. However, as a result of thecorrosion protection afforded by the oxidizing agent additive to theLithium electrode, the battery cell with the oxidizing agent exhibits alarger rate capability over time than the battery cell without theoxidizing agent. The benefits of oxidizing agent electrolyte additivesmay be useful for many types of battery systems and are not limited toLi/S battery cells described herein.

[0033] The use of oxidizing agent electrolyte additives may also beadvantageous when implemented in a battery cell with a lithium electrodeprotected with a thin Li-ion glassy conductor. The oxidizing agent tendsto penetrate into the cracks and other imperfections of the glassylayer. Then, the oxidizing agent reacts with the Li electrode protectingits surface from reacting with sulfur or polysulfides. In addition, thereaction product of lithium and the oxidizing agent formed on thesurface of the lithium electrode and within the cracks of the glassylayer may prevent further cracking of the glassy layer undergravitational and thermal stresses. Thus, the invented electrolyte mayprovide a mechanism for glassy layer healing in a battery cell employinga lithium electrode coated with a thin Li-ion glassy conductor.

[0034] Details and specific embodiments of the present invention inregards to oxidizer-treated lithium electrodes, battery cells containingsuch oxidizer-treated lithium electrodes, battery cell electrolytescontaining an oxidizing agent additive, methods of treating lithiumelectrodes with oxidizer and methods of assembling battery cellscontaining such oxidizer-treated lithium electrodes are described in thefollowing sections.

[0035] Battery Design

[0036] Suitable batteries may be constructed according to the known artfor assembling cell components and cells as desired, and any of theknown configurations may be fabricated utilizing the invention. Theexact structures will depend primarily upon the intended use of thebattery unit. Examples include thin film with porous separator, thinfilm polymeric laminate, jelly roll (i.e., spirally wound), prismatic,coin cell, etc.

[0037] The negative electrode is spaced from the positive sulfurelectrode, and both electrodes may be in material contact with anelectrolyte separator. Current collectors contact both the positive andnegative electrodes in a conventional manner and permit an electricalcurrent to be drawn by an external circuit. In a typical cell, all ofthe components will be enclosed in an appropriate casing, for example,plastic, with only the current collectors extending beyond the casing.Thereby, reactive elements, such as sodium or lithium in the negativeelectrode, as well as other cell elements are protected.

[0038] Conventional cell designs are known in the art, which may beconsulted for details. Examples of sulfur cells employing various designconfigurations are set forth in the following references which areincorporated herein by reference for all purposes: (1) R. D. Rauh, F. S.Shuker, J. M. Marston and S. B. Brummer, J. Inorg. Nuc. Chem.,“Formation of Lithium Polysulfides in Aprotic Media”, 39, 1761 (1977);(2) R. D. Rauh, K. M. Abraham, G. F. Pearson, J. K. Suprenant and S. B.Brummer, “A Lithium/Dissolved Sulfur Battery with an OrganicElectrolyte,” J. Electrochem. Soc., 126, 523 (1979); (3) H. Yamin, A.Gorenshtein, J. Penciner, Y. Sterberg, and E. Peled, “Lithium SulfurBattery,” J. Electrochem. Soc., 135, 1045 (1988); (4) H. Yamin and E.Peled, “Electrochemistry of a Nonaqueous Lithium/Sulfur Cell,” J. PowerSources, 9, 281 (1983); and (5) E. Peled, Y. Sterberg, A. Gorenshtein,and Y. Lavi, “Lithium-Sulfur Battery: Evaluation of Dioxolane-BasedElectrolyte,” J. Electrochem. Soc., 136, 1621 (1989).

[0039] Referring now to FIG. 2, a cell 10 in accordance with a preferredembodiment of the present invention is shown. Cell 10 includes anegative current collector 12, which is formed of an electronicallyconductive material. The current collector serves to conduct electronsbetween a cell terminal (not shown) and a negative electrode 14 (such aslithium) to which current collector 12 is affixed. If negative electrode14 is made from lithium or other similarly reactive material, it willpreferably include a protective layer 8 formed opposite currentcollector 12. Either negative electrode 14 or protective layer 8 (ifpresent) contacts a liquid electrolyte in an electrolyte region 16.

[0040] Region 16 may be delineated by the boundaries of a separator,which prevents electronic contact between the positive and negativeelectrodes. A positive electrode 18 abuts the side of separator layer 16opposite negative electrode 14. As electrolyte region 16 is anelectronic insulator and ionic conductor, positive electrode 18 isionically coupled to but electronically insulated from negativeelectrode 14. Finally, the side of positive electrode 18 oppositeelectrolyte region 16 is affixed to a positive current collector 20.Current collector 20 provides an electronic connection between apositive cell terminal (not shown) and positive electrode 18.

[0041] Current collectors 12 and 20, which provide current connectionsto the positive and negative electrodes, should resist degradation inthe electrochemical environment of the cell and should remainsubstantially unchanged during discharge and charge. In one embodiment,the current collectors are sheets of conductive material such asaluminum, copper, or stainless steel. The positive electrode may beattached to the current collector by directly forming on the currentcollector or by pressing a pre-formed electrode onto the currentcollector. Positive electrode mixtures formed directly onto currentcollectors preferably have good adhesion. Positive electrode films canalso be cast or pressed onto expanded metal sheets. Alternately, metalleads can be attached to the positive electrode by crimp-sealing, metalspraying, sputtering or other techniques known to those skilled in theart. The sulfur-based positive electrode can be pressed together withthe electrolyte separator sandwiched between the electrodes. In order toprovide good electrical conductivity between the positive electrode anda metal container, an electronically conductive matrix of, for example,carbon or aluminum powders or fibers or metal mesh may be used.

[0042] In the case where sulfur is provided entirely as a dissolvedspecies in the electrolyte, positive electrode 18 may primarily includean electronic conductor such as a carbon fiber matrix together with abinder or other additives. In the case where sulfur is provided in boththe solid and liquid (dissolved) phases, positive electrode 18 willinclude some amount of active sulfur in conjunction with the electronicconductor and possibly additives.

[0043] The separator may occupy all or some part of electrolytecompartment 16. Preferably, it will be a highly porous/permeablematerial such as a felt, paper, or microporous plastic film. It shouldalso resist attack by the electrolyte and other cell components underthe potentials experienced within the cell. Examples of suitableseparators include glass, plastic, ceramic, and porous membranes thereofamong other separators known to those in the art. In one specificembodiment, the separator is Celgard 2300 or Celgard 2400 available fromHoechst Celanese of Dallas, Tex.

[0044] The separator may also be of the type sometimes referred to as a“polymer” separator membrane having a porous or microporous network forentraining liquid electrolyte. Such separators are described in U.S.Pat. No. 3,351,495 assigned to W. R. Grace & Co. and U.S. Pat. Nos.5,460,904, 5,540,741, and 5,607,485 all assigned to Bellcore, forexample. These patents are incorporated herein by reference for allpurposes.

[0045] In a preferred embodiment, the metal-sulfur cell has relativelythick positive and negative electrodes. Such thick format cells areparticularly beneficial for use as primary cells, although they areappropriate in some secondary cells as well. The additional materialprovided in the electrodes provides extra capacity for long life. In aspecific example, the positive electrode of a primary cell has anaverage thickness of at least about 40 micrometers and the positiveelectrode of a secondary cell has an average thickness of at most about8 micrometers. These thickness' apply to electrodes in which all sulfuris provided by the catholyte and those in which at least some sulfur isprovided in the solid phase and localized at the electrode. Generallysuch thickness' represent a bare minimum and are appropriate with veryhigh concentration catholytes (e.g., 15 molar sulfur or possibly even 25molar sulfur). In more typical embodiments, the positive electrodethickness will range between about 40 and 130 micrometers for primarycells and between about 8 and 30 micrometers for secondary cells. Notethat it may be feasible and appropriate to provide sulfur as asuspension (e.g., a colloid) in the catholyte. Note also that it isgenerally desirable that the positive electrode have a relatively highporosity, possibly as high as 95% or more. Generally, higher porosityelectrodes allow fabrication of cells with higher laminate energydensities because less electronic conductor is required. Of course, anelectrode's porosity, capacity, and thickness are linked so that settingtwo of these parameters fixes the other.

[0046] Electrochemical and Chemical Mechanisms of Lithium-Sulfur LiquidElectrolyte Batteries

[0047] Referring again to FIG. 2, lithium-sulfur cell 10 will bedescribed with relevant reaction mechanisms explained. During normalcharging, the electrons are extracted from positive electrode 18 andtransported over electrical connection 38 to negative electrode 14. Theremoval of electrons at positive electrode 18 oxidizes the speciespresent in the electrode. In this reaction, lithium ions are liberatedfrom lithium sulfide and/or lithium polysulfide species present in thepositive electrode. The species remaining in the positive electrode willhave the general formula Li₂S_(x), where x has a value of 1 or greater.Over time the charge reaction produces polysulfide species having longerand longer sulfur chains. It is known for example that in a normalcharge reaction, the value of x in some polysulfides may be 12 orgreater. In addition, some of the polysulfides will be further oxidizedto elemental sulfur.

[0048] At the negative electrode, lithium ions present in theelectrolyte 16 are transported through protective layer 8 and reduced tolithium metal as electrons are moved through electrical conduit 38.

[0049] The above electrode reactions proceed in the reverse directionduring discharge. That is, the electrochemical reduction of activesulfur pulls electrons to positive electrode 18 through currentcollector 20 and from line 38. This reduces elemental sulfur, ifpresent, to form various lithium species including lithium polysulfidesand lithium sulfide. It also reduces the highly oxidized polysulfides toless oxidized polysulfides and lithium sulfide. Simultaneously, lithiumions are provided from negative electrode 14 through the electrolyte.The lithium ions are generated in conjunction with the flow of electronsfrom negative electrode 14 to line 38 (via current collector 12).

[0050] Generally, the higher molecular weight polysulfides (those withmore sulfur atoms) are more highly soluble than their lower molecularweight counterparts. During discharge, these higher molecular weightspecies go into solution in the electrolyte and migrate throughout thecell. Some of the dissolved species move to the negative lithium metalelectrode where they may be chemically reduced to form less-solublelower molecular weight compounds such as lithium sulfide. Some of thislithium sulfide may form as a layer on the lithium metal electrode. Thelithium sulfide layer may affect the batteries discharge characteristicssuch as polarization. Further, excess lithium sulfide may precipitateout of solution where it serves no beneficial use in the cell. In fact,the precipitated lithium sulfide (and/or less-soluble lower molecularweight polysulfides) represents lithium and sulfur that is no longeravailable for immediate participation in electrochemical reactions.Thus, precipitation of these compounds reduces the battery's capacity.

[0051] Precipitated sulfide or polysulfide may also form because thelocal solution concentration of these species exceeds their solubilitylimits. This occurs when the species are generated faster than they candiffuse away, a condition that exists when the local current density istoo great in comparison with the mass transport rate. That is, thesolution phase concentration gradient must support a mass flux that issufficiently high to remove reaction products before they accumulate totheir solubility limit. The present invention addresses this problem inat least two ways. First, it provides electrolyte solvents in which thedischarge species are highly soluble and highly mobile thereby reducingthe likelihood of precipitation. Second, it provides a cathode structurein which the mass flux is sufficiently fast that the local concentrationof soluble species does not exceed the solubility limits.

[0052] Assuming that some precipitation will occur so that solid phasesulfur, sulfide, and/or polysulfide exist in the cell, it is importantthat the cell be designed to make these precipitated electroactivespecies available to electronic and ionic charge carriers. This allowshigh utilization of the active sulfur in the cell. To this end, theelectronic conductor in the positive electrode should form aninterconnected matrix so that there is always a clear current path fromthe positive current collector to any position in the electronicconductor. The interconnected matrix of the electronic conductor shouldalso be sufficiently “open” that there is room for precipitatedelectroactive species to deposit on the matrix. Finally, any binderemployed in the positive electrode should not prevent contact betweenthe electronic conductor and the electroactive species. For example, thebinder should not provide so much wetting that precipitated sulfurparticles and/or the current collector are completely wetted andtherefore unable to exchange electrons.

[0053] LIQUID ELECTROLYTES

[0054] It has now been discovered that the performance of lithium-sulfurbatteries can be improved by employing electrolyte compositions designedto solubilize lithium sulfide and relatively low molecular weightpolysulfides. The new electrolytes of this invention contain oxidizingagent additives that provide an in-situ mechanism for forming aprotective coating on the metal electrode when the additive is used in abattery cell. In general, the electrolyte compositions of this inventioninclude one or more solvents that strongly coordinate lithium. Thesesolvents are “ionophores” for lithium ions. Exemplary ionophores arepodands, coronands, and cryptands as described in chapter 10 of thetreatise “Lithium Chemistry, A Theoretical and Experimental Overview,”Anne-Marie Sapse and Paul Von Rague Schleyer, Eds. John Wiley & Sons,New York (1995) which is incorporated herein by reference for allpurposes. Chapter 10 was written by Bartsch et al. Podands are acyclicmultidentate ligands. Common examples are glymes and polyethyleneglycols. Coronands are monocyclic multidentate ligands. Common examplesare crown ethers and substituted crown ethers. Cryptands are multicyclicmultidentate ligands.

[0055] In a preferred embodiment, the electrolyte solvents of thisinvention include one or more compounds having an ethanediether linkage.They have the general formula R₁(CH₂CH₂O)_(n)R2, where n ranges between1 and 10 and R1 and R2 are different or identical alkyl or alkoxy groups(including substituted alkyl or alkoxy groups). Alternatively, R1 and R2may together form a closed ring to form a crown ether for example.Examples of linear solvents include the glymes (CH₃O(CH₂CH₂O)_(n)CH₃)including monoglyme, diglyme, triglyme and tetraglyme and related oxidesof the formula (CH₂CH₂O)_(n)(CH₂O)_(p), where p ranges from about 1 to50. Such ethoxy repeating unit compounds serve as lithium ioncoordinating solvents. In a preferred embodiment, the main solvent is aglyme having a value of n ranging between 1 and 6. In an especiallypreferred embodiment, the glyme is 1,2-dimethoxyethane(CH₃O(CH₂CH₂O)CH₃). In another embodiment, the main solvent is a linearpolyether.

[0056] Oxidizing agents, which may be added to the electrolyte solventsof this invention include soluble active additives. At least one of thefollowing compounds is added to the electrolyte solvents including,sulfur dioxide (SO₂), nitrous oxide (N₂O), carbon dioxide (CO₂),halogens, interhalogens, oxychlorides, sulfur monochlorides, vinylcarbonate and halogenated carbonates. Halogens (Cl₂, Br₂ and I₂) as wellas oxychlorides (SO₂Cl₂ and SOCl₂), and sulfur monochlorides S₂Cl₂ havestronger oxidizing ability than elemental S. The potentials of thesecompounds versus Li exceed 3.5 V. The Li—I₂ couple OCV is about 2.8V. Ina battery cell fabrication, it may be more practical to useinterhalogens (e.g. iodine monochloride (ICl), iodine trichloride (ICl₃)and iodine monobromide I₂Br₂) instead of halogens because interhalogensare liquids or solids that can easily dissociate in the electrolyte andform halogens. Also, SO₂ and N₂O are stronger oxidizers than thepolysulfides or the elemental S. For example, the Li-SO₂ couple OCV isabout 3.0 V.

[0057] Strong oxidizing agents in the electrolyte can serve two mainfunctions when incorporated into a Li/S battery cell. First, because oftheir strong oxidizing ability the oxidizers immediately react with Liforming a solid passivating film with Li-ion conductivity. If thematerial of the passivating film has a low solubility in the electrolytesolution, the Li electrode is protected from reaction with other activecomponents of the electrolyte. For example, the Li electrode may beprotected from reactions with dissolved elemental sulfur andpolysulfides. In the case of SO₂, a protective layer of the Li-ionconductor Li₂S₂O₄ is formed. In the case of additives of thionylchlorideand iodine trichloride, a polycrystalline layer of LiCL with arelatively high ionic conductivity is formed. Second, during cellstorage under OCV conditions, the additives may maintain the highestpossible state of sulfur oxidation because of oxidation of thepolysulfides to the elemental sulfur. Further, as described above, theaddition of oxidizing agents in the electrolyte may add additionalbattery capacity because the additives are stronger oxidizers than thepolysulfides

[0058] The batteries of this invention can operate at room temperature.However, the present invention also pertains to systems operating attemperatures slightly outside of the ambient. Obviously, the choice ofoperating temperature ranges can influence the preferred electrolyte forthe batteries of this invention. For example, at relatively lowoperating temperatures, lower molecular weight electrolytes will bepreferred, so the value of “n” in the above-describe ethanediethercompounds will be in the lower end of the 1-10 range. At highertemperatures, the opposite is true.

[0059] While the electrolyte solvents described above are a maincomponent of the electrolytes of this invention, one or more cosolventsmay be provided with them. If such cosolvents are employed, they arepreferably chosen to solubilize lithium cations or sulfide/polysulfideanions. In certain preferred embodiments, crown ethers and/or cryptandsare provided as cosolvents. In other preferred embodiments, donor oracceptor cosolvents may be employed. In a particularly preferredembodiment, the cosolvent is dioxolane. Preferred dioxolane-containingelectrolytes are further described in U.S. patent application Ser. No.09/245,167, titled DIOXOLANE AS A COSOLVENT IN Li/Li2Sx BATTERIES, filedon Feb. 5, 1999, and having Nimon et al. as inventors. That document isincorporated herein by reference for all purposes.

[0060] Crown ethers also can be used as co-solvents. Crown ethers aremacrocyclic polyethers generally having repeating ethoxy and/or propoxygroups. Crown ethers with 3 to 20 oxygen atoms have been synthesized.They are typically made up of linked ethoxy units (CH₂CH₂O)n as shownbelow. A general abbreviation used is n-C-m where n is the ring size andm is the number of oxygen atoms in the ring.

[0061] Commercially available crown ethers which have application inthis invention include 12-crown-4, 15-crown-5, 18-crown-6, and modifiedcrowns such as dibenzo-18-crown-6. Crown ethers are known to becomplexing agents which solubilize alkali metal cations in nonpolarsolvents. 12-crown-4 is known to be specific for the lithium cation.

[0062] In substituted crown ethers, one or more of the hydrogen atomsare replaced with a hydrocarbon group that may be linear, branched, oraromatic. These hydrocarbon groups may, in turn, be substituted withhalo (F, Cl, Br, I), nitrile (CN), nitro (NO₂), hydroxy (OH), and othercommon substituent groups. Examples are presented in chapter 10 of theabove referenced “Lithium Chemistry” treatise. Specific examples includedibenzo-14-crown-4, tetramethyl-12-crown-4, benzo-15-crown-5. Crownethers may also be substituted with podand groups such as(—COCH₂CH₂OCH₃) to form podano-coronands or “lariat ethers.”

[0063] In an alternative embodiment, the main solvent is a cryptand.Cryptands are also known to strongly complex with alkali metal cations.Structurally, they are similar to crown ethers but possess an additional(—XCH₂CH₂) bridge to create an additional ring. X may be an oxygen,nitrogen, or sulfur atom. Often X is nitrogen and the correspondingcryptands are described as containing two nitrogen atoms linked by three(CH₂CH₂O)n bridges. These compounds are commonly identified by thenumber of oxygen atoms in each of the three bridges. Thus, a cryptand inwhich two of the bridges have n=2 (two oxygen atoms) and a third bridgehaving n=1 (one oxygen atom) is identified as [2.2.1]-cryptand.

[0064] One example of a general class of cosolvents is the donorsolvents, which tend to solubilize cations and acceptor solvents whichtend to solubilize anions. Donor solvents are characterized by highdonor numbers DN. A desirable property of both donor and acceptorcosolvents used in this invention is a high dielectric constant. Suchsolvents generally promote dissociation of an ionic solute or a contaction-pair.

[0065] Generally, donor solvents are those solvents which can becharacterize as Lewis bases (they may be aprotic solvents). Generally,these solvents are good at solvating cations such as lithium ions. Donorsolvents promote the ionization of covalent compounds to form intimate(or contact) ion-pairs. The concept of a solvent donor number is furtherexplained and exemplified in “Experimental Electrochemistry forChemists,” by Sawyer and Roberts, Jr., John Wiley & Sons, New York(1995). That reference is incorporated herein by reference for allpurposes.

[0066] Suitable donor cosolvents include hexamethylphosphoramide,pyridine, N,N-diethylacetamide, N,N-diethylformamide, dimethylsulfoxide,tetramethylurea, N,Ndimethylacetamide, N,N-dimethylformamide,tributylphosphate, trimethylphosphate, N,N,N′,N′-tetraethylsulfamide,tetramethylenediamine, tetramethylpropylenediarnine, andpentamethyldiethylenetriamine. These assist in solvation of lithiumions.

[0067] Suitable acceptor solvents assist in solvation of the sulfide andpolysulfide anions. Acceptor solvents are those solvents which can becharacterized as Lewis acids (they may be aprotic or aprotic solvents)and promote solvation of anions. Examples include alcohols such asmethanol, glycols such as ethylene glycol, and polyglycols such aspolyethylene glycol, as well as nitromethane, trifluoroacetic acid,trifluoromethanesulfonic acid, sulfur dioxide, and boron trifluoride.

[0068] It should be understood that the electrolyte solvents of thisinvention may also include other cosolvents which do not necessary fallinto the donor solvent and acceptor solvent classes. Examples of suchadditional cosolvents include sulfolane, dimethyl sulfone, dialkylcarbonates, tetrahydrofuran (THF), dioxolane, propylene carbonate (PC),ethylene carbonate (EC), dimethyl carbonate (DMC), butyrolactone,N-methylpyrrolidinone, dimethoxyethane (DME or glyme), and combinationsof such liquids.

[0069] In general, the liquid electrolyte solvents of this inventioninclude about 50 to 100% by weight of the main solvent (excluding salts)which is usually one or more podand such as the above-describedethanediether compounds and include up to 49% by weight of the oxidizingadditive. The balance will be one or more of the cosolvents listedabove. More preferably, the electrolyte solvents include about 50 to100% by weight main solvent, and most preferably between about 70 and90% by weight main solvent. As noted, the main solvent is one or more ofthe lithium coordinating ionophores described above (podands such asglymes, coronands such as crown ethers, or cryptands). Aside from themain solvent, the electrolyte solvent may include one or more cosolvents(described above) which make up the balance. In a particularly preferredembodiment, the electrolyte solvent includes DME as the main solvent anddioxolane as the cosolvent, with the dioxolane making up between about 5and 15% by weight of the mixture.

[0070] Exemplary but optional electrolyte salts for the battery cellsincorporating the electrolyte solvents of this invention include, forexample, lithium trifluoromethanesulfonimide (LiN(CF₃SO₂)₂), lithiumtriflate (LiCF₃SO₃), lithium perchlorate (LiClO₄), LiPF₆, LiBF₄, LiAsF₆,LiN(C₂F₅SO₂)₂, as well as, corresponding salts depending on the choiceof metal for the negative electrode, for example, the correspondingsodium salts. As indicated above, the electrolyte salt is optional forthe battery cells of this invention, in that upon discharge of thebattery, the metal sulfides or polysulfides formed can act aselectrolyte salts, for example, Mx/zS wherein x=0 to 2 and z is thevalence of the metal.

[0071] Regardless of whether the sulfur is present in a solid phase, thecells of this invention preferably operate with their electrolytes at aconcentration of between about 3 and 30 molar sulfur, more preferablybetween about 5 and 25 molar sulfur, and most preferably between about10 and 20 molar sulfur. The sulfur used in this measure is the sulfuratoms in electroactive species. Thus, for example, one molar Li₂Scorresponds to one molar sulfur, whereas one molar Li₂S₅ corresponds tofive molar sulfur, and one molar S₈ corresponds to eight molar sulfur.Note that in some cases at least some of the sulfur may exist assuspended particles within the catholyte.

[0072] It should be understood that some systems employing liquidelectrolytes are commonly referred to as having “polymer” separatormembranes. Such systems are considered liquid electrolyte systems withinthe context of this invention. The membrane separators employed in thesesystems actually serve to hold liquid electrolyte in small pores bycapillary action. Essentially, a porous or microporous network providesa region for entraining liquid electrolyte. As mentioned above, suchseparators are described in U.S. Pat. No. 3,351,495 assigned to W. R.Grace & Co. and U.S. Pat. Nos. 5,460,904, 5,540,741, and 5,607,485 allassigned to Bellcore, for example.

[0073] The battery cells of this invention may also include a gel-stateor a solid-state electrolyte. An exemplary solid-state electrolyteseparator is a ceramic or glass electrolyte separator which containsessentially no liquid. Specific examples of solid-state ceramicelectrolyte separators include beta alumina-type materials such assodium beta alumina, Nasicon™ or Lisicon™ glass or ceramic. Polymericelectrolytes, porous membranes, or combinations thereof are exemplary ofa type of electrolyte separator to which an aprotic organic plasticizerliquid can be added according to this invention for the formation of asolid-state electrolyte separator containing less than 20% liquid.Suitable polymeric electrolytes include polyethers, polyimines,polythioethers, polyphosphazenes, polymer blends, and the like andmixtures and copolymers thereof in which an appropriate electrolyte salthas optionally been added. Preferred polyethers are polyalkylene oxides,more preferably, polyethylene oxide. In addition, the electrolyteseparator could contain less than 20% by weight of adioxolane-containing liquid electrolyte, such as described above.

[0074] In the gel-state, the electrolyte separator contains at least 20%by weight of an organic liquid (see the above listeddioxolane-containing liquid electrolyte compositions for examples), withthe liquid being immobilized by the inclusion of a gelling agent. Manygelling agents such as polyacrylonitrile, PVDF, or PEO can be used.

[0075] In addition, oxidizing electrolyte additives in accordance withthe present invention may be used to supplement the protection affordedto glass-coated lithium electrodes, such as described incommonly-assigned co-pending U.S. patent application Ser. No. 09/431,190entitled “Encapsulated Lithium Alloy Electrodes Having Barrier Layers”,filed Nov. 01, 1999, the entire specification of which is incorporatedherein by reference. In the case of lithium electrodes coated with aprotective glass layer, the oxidizing electrolyte additives inaccordance with the present invention may effectively heal cracks whichform in the glass during cycling by penetrating the cracks andcontacting the lithium to form a protective coating. An advantage ofusing the oxidizing additive, in the present invention, with theelectrode covered with the glassy layer is that it reduces the qualityrequirements of the glassy layer and the complexity of quality controlneeded to manufacture the glassy electrode. In addition, theself-healing nature of the coating improves battery reliability as well.

[0076] negative electrode

[0077] Most generally, the negative electrode can comprise any metal,any mixture of metals, glass, carbon or metal/carbon material capable offunctioning as a negative electrode in combination with the sulfur-basedcomposite positive electrode of this invention. Accordingly, negativeelectrodes comprising any of the alkali or alkaline earth metals ortransition metals (the polyether electrolytes are known to transportdivalent ions such as Z_(n) ⁺⁺), for example, in combination with thepositive electrodes and electrolytes of this invention are within theambit of the invention, and particularly alloys containing lithiumand/or sodium.

[0078] Stated another way, the negative electrodes employed in thebatteries of this invention may include a metal (in elemental or alloyform) or an ion of that metal as used in, for example, a carbonintercalation electrode or a glass matrix electrode. As explained above,metal ions from this negative electrode combine with elemental sulfur orpolysulfides to produce a sulfide and polysulfides of the metal duringdischarge of the battery. Some of the resulting metal sulfides andpolysulfides may remain localized near the positive electrode. Somefraction of these discharge products, however, will dissolve in thesolvent of the electrolyte and move freely through the liquid containingregions of the cell. As mentioned, some of these dissolved dischargeproducts may actually precipitate from solution and become unavailablefor further electrochemical reaction, thereby reducing the cell'scapacity.

[0079] In one preferred embodiment, the materials for the negativeelectrodes include a metal such as lithium or sodium or an alloy of oneof these with one or more additional alkali metals and/or alkaline earthmetals. Preferred alloys include lithium aluminum alloys, lithiumsilicon alloys, lithium tin alloys, and sodium lead alloys (e.g.,Na₄Pb). Other metallic electrodes may include alkaline earth electrodessuch as magnesium and their alloys; transition metal electrodes such asaluminum, zinc, and lead and their alloys;

[0080] The surface of such metallic negative electrodes can be modifiedto include a protective layer on the electrolyte side. This protectivelayer should be conductive to lithium ions and help prevent theformation of lithium dendrites or “mossy” lithium on repeated cycling.As described above, it can be produced in situ of the battery cell bythe action of oxidizing additives, including sulfur dioxide, nitrousoxide, carbon dioxide, halogens, interhalogens, oxychlorides and sulfurmonochlorides. In addition, a protective coating may be added to anelectrode by pretreating the electrode with gaseous sulfur dioxide,carbon dioxide or halogens or pretreating the electrode with liquidoxychlorides. For example, prior to battery assembly, the lithiumelectrode may be pretreated with gaseous sulfur dioxide to produce afilm of Li₂S₂O₄ on the surface of the electrode. The film, as describedabove, provides the benefits such as improved discharge capacity afterstorage.

[0081] Examples of preferred protective layer formats and materials aredescribed in the following U.S. Patent applications, each of which isincorporated herein by reference for all purposes: U.S. patentapplication Ser. No. 09/086,665, filed May. 29, 1998, titled PROTECTIVECOATINGS FOR NEGATIVE ELECTRODES, and naming Visco et al. as inventors;U.S. patent application Ser. No. 09/139,603, filed Aug. 25, 1998, titledPLATING METAL NEGATIVE ELECTRODES UNDER PROTECTIVE COATINGS, and namingChu et al. as inventors; U.S. patent application Ser. No. 09/139,601,filed Aug. 25, 1998, titled METHOD FOR FORMING ENCAPSULATED LITHIUMELECTRODES HAVING GLASS PROTECTIVE LAYERS, and naming Visco et al. asinventors; and US Patent Application No. 09/431,190, filed Nov. 1, 1999,titled ENCAPSULATED LITHIUM ALLOY ELECTRODES HAVING BARRIER LAYERS, andnaming Chu et al. as inventors.

[0082] In an alternative embodiment, the negative electrode may be anintercalation electrode such as a carbon-based lithium ion electrode.Such electrodes are available in commercial lithium ion batteriesavailable from Sony Corporation of Japan. These materials are describedby Jeffrey Dahn in Chapter 1 of “Lithium Batteries, New Materials,Developments and Perspectives,” edited by G. Pistoia and published byElsevier (1994), which reference is incorporated herein by reference.Generally, such electrodes have the formula Li_(y)C₆ (where y=0.3 to 2).For many of these materials, the fully charged electrode has the formulaLiC₆. The intercalation electrode matrix may include graphite, petroleumcoke, carbon inserted within highly disordered carbons, etc. Theinserted carbon may also be doped with boron, phosphorus, or otherappropriate dopant. In one example, the carbon may be prepared from lowtemperature pyrolysis (about 750° C.) of carbon or carbon-siliconcontaining polymers such that the carbon product retains some hydrogenor silicon or both. (See, Sato et al., “A Mechanism of Lithium Storagein Disordered Carbons,” Science, 264: 556 (22 Apr. 1994), whichdiscusses very good results with a preferred negative electrode of Liinserted within poly p-phenylene-based carbon).

[0083] Glass matrix negative electrodes such as Li/Sn₂O₃ and Li/SiO₂ mayalso be employed in the batteries of the present invention. Theseelectrodes are similar to the above-described carbon-based intercalationelectrodes in that lithium ions are inserted therein during charge andremoved during discharge. Such glass matrix electrodes are described invarious references including Tahara et al., European Patent ApplicationNo. 93111938.2 (1993), Idota et al. Canadian Patent Application,21134053 (1994), and I. Courtney et al. Meeting Abstracts of theElectrochemical Society, Fall Meeting, San Antonio, Tex., Oct. 6-11,1996 Vol. 96-2, Abstract # 66, page 88, each of which is incorporatedherein by reference for all purposes.

[0084] FIGS. 3A-B illustrate a fabrication process for a glassy coatedlithium electrode which may be used with an oxidizing agent electrolyteadditive of the present invention. Considering FIG. 1A first, a lithiumelectrode 110 is fabricated as a laminate in the following manner.Optionally, a thin layer of a release agent 112 is deposited on a webcarrier 114 by evaporation for example. This web carrier and the releaseagent should have a very smooth surface. Deposition of the release agentis followed by deposition of a single ion conductor 116 onto releaseagent 112 by a suitable process such as sputtering, chemical vapordeposition, coating, extrusion/calendering, or spray coating. Layer 116serves as a barrier layer in the completed electrode and is thereforepreferably a single ion conductor which conducts ions of the activemetal used in the electrode (e.g., lithium). Because barrier layer 116is deposited on a very smooth surface, it too will be smooth andcontinuous.

[0085] As described above, any cracks that may form in the barrier layer116 may be filled (e.g. “healed”) by a reaction product of the activemetal and an oxidizing agent additive in the electrolyte. For example,when a SO₂ oxidizing additive is part of the electrolyte solution of abattery cell with a glass coated lithium electrode ,any cracks in theglass coat may allow SO₂ to react with the surface of the electrode.When the SO₂ reacts with Li in the electrode, Li₂S₂O₄ is formedessentially sealing the cracks in the glassy layer.

[0086] Next, after the barrier layer is formed, a bonding layer 117 isformed on barrier layer 116. This material should easily and stronglybond with the active metal. Preferably it is also substantiallynon-reactive with ambient agents such as moisture and other gases inair. Aluminum and aluminum alloys work well as bonding layer materialswhen lithium is the active metal.

[0087] Lithium 118 (or other active metal for the electrode) isdeposited on bonding layer 117 by evaporation for example. Then, acurrent collector 120 (e.g., a copper layer of about 1000 angstroms toone micrometer thickness) is optionally formed on lithium layer 118 by aconventional process such as evaporation or sputtering. Finally, thebarrier layer/lithium layer/current collector laminate is separated fromthe carrier 114, with release layer 112 giving way. The bonding layerfacilitates adherence of the lithium to the barrier layer to allow cleanseparation without damaging the barrier layer.

[0088] The resulting structure may be referred to as an “encapsulatedelectrode.” Because the lithium is encapsulated within the barrier layerand the current collector, it may be transported, stored, and otherwisehandled without the precautions normally required for a lithium metalelectrode. Note that in some embodiments, a current collector is notemployed. Rather, the layer of lithium is protected on one side andexposed on the other.

[0089] It is possible that the barrier layer laminate can be produced ina continuous fashion. A fresh layer is formed on the web as it passesthrough each of a series of stations. The barrier layer laminate,including web, the release agent, the barrier layer, and the bondinglayer, may be stored under ambient conditions.

[0090] Because the web carrier supports continuous fabrication of theelectrode laminate through a series of deposition reactors, it shouldwithstand high temperatures and wide pressure ranges. Examples ofsuitable web materials include plastics such as polyethyleneterephthalate (PET), polypropylene, polyethylene, polyvinylchloride(PVC), polyolefins, and polyimides. The web carrier should have athickness and tensile strength suitable for web handling at the linespeeds dictated by the metal and glass or polymer deposition steps.

[0091] The release agent serves to release the subsequently formedelectrode from the web carrier. The particular release layer chosendepends upon the types of web carrier and barrier layer employed.Suitable release agents are known in the art. In a specific embodiment,the release layer is a 50 angstrom copper film formed by evaporation orsputtering. The release agent should be as thin as possible while stillretaining release properties, and easily dissolving in the targetbattery environments. In the case of a copper release, a thick copperrelease film could potentially block ionic transport to the barrierlayer. Therefore a thin Cu layer is envisaged whereby, once in thebattery environment, the thin copper layer is removed by corrosionand/or dissolution, exposing the barrier layer to the batteryelectrolyte.

[0092] The encapsulated electrode 110 resulting from this processincludes a lithium metal layer 118 sandwiched between current collector120 and barrier layer 116. Because the lithium layer is formed after thebarrier layer (rather than having the barrier layer deposited on apotentially rough lithium surface as in conventional processes), thebarrier layer is of high quality. That is, the barrier layer isgenerally gap-free and adherent when produced according to thisinvention. As mentioned, it may be difficult to directly deposit glassonto a lithium film due to the high degree of surface roughness of thelithium film relative to the sputter deposited glass film thickness(e.g., 300 to 1500 angstroms).

EXAMPLES

[0093] Various experiments were conducted to demonstrate the performanceadvantages provided by the various aspects of this invention. It shouldbe understood that the experiments described in the following examplesare representative only and in no way limit the scope of the presentinvention. The experiments are provided mainly to show the high level ofperformance that can be attained when following the guidelines presentedherein.

[0094] Laboratory electrochemical cells of two types were constructed:with carbon cathodes (Li/C cells) and with elemental sulfur-loadedcathodes (Li/S cells).

[0095] Li/S CELLS

[0096] The Li/S cell contained a lithium anode (lithium foil 125 umthick from AVESTOR, Montreal, Canada), a porous sulfur-loaded cathode,and an electrolyte solution containing a supporting salt,LiN(SO₂CF₃)₂—lithium bistrifluoro-methane-sulfonilimide (LiTFSI),dissolved in a mixture of 1,2 dimethoxyethane (DME) with 1,3 dioxolane.The supporting salt was used at a concentration of 0.5 mole per liter ofsolution. The ratio of DME to dioxolane was 9:1 by weight. Allelectrolyte components were bought from Aldrich Chemical Company, Inc.,Milwaukee, Wis.

[0097] The separator was a micro-porous polymeric layer having a nominalthickness of 25 microns (Hoechst Celanese, Celgard 2400). The separatorwas vacuum dried overnight prior to transfer into an argon-filled glovebox.

[0098] The sulfur-loaded cathode was made by impregnating a slurrycontaining elemental sulfur into a carbon fiber paper (Lydall Technicalpapers, Rochester, N.Y.). The slurry composition was 50 wt % sulfur, 28wt % carbon black, 20 wt % polyethylene oxide (MW=900K), and 2 wt %Brij35 dissolved in acetonitrile. The cathode was vacuum dried overnightprior to transfer into the glovebox for final cell assembly.

[0099] 230 microliters of the electrolyte were placed on the porouscathode followed by placement of the microporous separator on thecathode. An additional 20 microliters of electrolyte were then placed onthe separator layer. Once assembled, the cell was compressed at 2 psi.

[0100] Li/C CELLS

[0101] The Li/C cell contained a lithium anode (lithium foil 125um thickfrom AVESTOR, Montreal, Canada), a porous carbon cathode, and anelectrolyte solution containing 0.5 mole per liter of LiTFSI and thelithium polysulfide Li₂S₈ completely dissolved in the mixture of DME andDioxolane (9:1).

[0102] The carbon porous cathode was made by air spraying a slurrycontaining 70% carbon black and 30% polyvinylidene flouride (Mw˜250,000)into a carbon fiber paper (Lydall Technical papers, Rochester, N.Y.).The solvent was Dimethyl formamide. The slurry contained approximately11% solids by weight. The cathode was vacuum dried overnight prior totransfer into the glovebox for cell assembly. 330 microliters of theelectrolyte containing dissolved polysulfides were placed on the porouscathode followed by placement of the microporous separator on thecathode. An additional 20 microliters of electrolyte were then placed onthe separator layer. Once assembled, the cell was compressed at 2 psi.

[0103] The cells were tested at 25° C. with a Series 4000 battery testsystem from Maccor Inc. of Tulsa, Okla.

Example 1

[0104] Li/S cells with S-loaded cathodes were discharged at 1.0 mA/cm²after 5 days of storage. A first cell contained an SO₂ additive in theelectrolyte while a second cell did not contain the SO₂ additive.Besides the different electrolyte formulations, the first cell and thesecond cell were essentially similar. The SO₂ additive in the first cellcomprised 3.2% of the electrolyte by weight. The second cell without theSO₂ additive in the electrolyte lost more than half of its initialcapacity after 5 days of storage. The first cell having the electrolytewith 3.2% by weight of dissolved SO₂ exhibited a much larger capacity ascompared to the second cell without the SO₂ additive (See FIG. 4).

Example 2

[0105] Li/C cells with 12 M sulfur as Li₂S₈ dissolved in the mixture ofDME and Dioxolane (9:1) were discharged at 1.0 mA/cm² after 4 days ofstorage. A lithium electrode in a first cell was pretreated with SO₂ gasprior to assembly of the first battery cell while a lithium electrode ina second cell was not pretreated with the SO₂ gas. Besides pre-treatmentof the lithium electrode with SO₂ gas, the first cell and the secondcell were essentially similar. The second cell with the untreatedlithium electrode quickly polarized. The first cell having the lithiumelectrode pretreated with SO₂ gas exhibited a much larger capacity ascompared to the second cell with the lithium electrode not treated withSO₂ gas (See FIG. 5).

Example 3

[0106] Li/S cells with S-loaded cathodes were discharged at 1.0 mA/cm²after 5 days of storage. A lithium electrode in a first cell waspretreated with SO₂ gas prior to assembly of the first battery cellwhile a lithium electrode in a second cell was not pretreated with theSO₂ gas. In addition, a first cell contained an SO₂ additive in theelectrolyte while a second cell did not contain the SO₂ additive.Besides the different electrolyte formulations and pre-treatment of thelithium electrode with SO₂ gas, the first cell and the second cell wereessentially similar. The SO₂ additive in the first cell comprised 3.0%of the electrolyte by weight. The first cell having the lithiumelectrode pretreated with SO₂ gas and SO₂ electrolyte additive exhibiteda much larger capacity as compared to the second cell with the lithiumelectrode not treated with SO₂ gas and without SO₂ electrolyte additive(See FIG. 6).

Example 4

[0107] An impedance of the interface between the Li electrode and theelectrolyte containing dissolved lithium polysulfides was measured inthe symmetrical cell with two identical Li electrodes. The solution was7.5 M sulfur as Li₂S₈ dissolved in the mixture of DME and Dioxolane(9:1) with 0.5 M LiTSFI. Impedance measurements were performed duringcell storage under open circuit conditions for different time periods. Alithium electrode in a first cell was pretreated with liquid thionylchloride for 1.5 hourses prior to assembly of the cell while a lithiumelectrode in a second cell was not pretreated with the thionyl chloride.Besides pre-treatment of the lithium electrode with the liquid thionylchloride, the first cell and the second cell were essentially similar.The lithium electrode pretreated with thionyl chloride exhibited low andstable interface resistance. The interface resistance of the untreatedlithium electrode grew rapidly during storage. An increasing interfaceresistance is typically consistent with corrosion of the lithiumelectrode via reaction with the long chain polysulfides dissolved in thesolvent mixture. The stable interface resistance of the electrodepretreated with the liquid thionyl chloride indicates the formation of apassivating film that protects the Li electrode from reacting with thepolysulfides. After 10 days of storage the interface resistance of thetreated electrode is about five times less than the resistance of theuntreated one (See FIG. 7).

Conclusion

[0108] The use of oxidizing agent as a protector for lithium electrodeshas been described. Battery cells with the electrolyte containingdissolved oxidizing additives or electrodes pretreated with the oxidizerand then incorporated into the battery cell exhibit improvedstorageability and increased discharge capacity over cells withoutoxidizing additives in the electrolyte or without electrodes pretreatedwith the oxidizing additive. The present invention is applicable to bothprimary and rechargeable lithium-sulfur batteries. In addition, methodsand compositions in accordance with the present invention, includingstrong oxidizer-pretreated electrodes and electrolytes containingoxidizing additives , may be used in conjunction with other lithiumbattery cells and fabrication techniques. All references cited in thisapplication are incorporated by reference for all purposes.

[0109] Although the foregoing invention has been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the appended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A battery cell electrolyte, comprising: a) a mainsolvent of an electrolyte solvent mixture, having the chemical formulaR₁(CH₂CH₂O)_(n)R₂, where n ranges between 1 and 10, R₁ and R₂ aredifferent or identical groups selected from the group consisting ofalkyl, alkoxy, substituted alkyl, and substituted alkoxy groups; and b)an oxidizing agent additive comprising no more than about 49% by weightof the electrolyte solvent mixture.
 2. The electrolyte of claim 1,wherein the oxidizing agent additive is at least one of sulfur dioxide,nitrous oxide, carbon dioxide, a halogen, an interhalogen, anoxychloride, a sulfur monochloride, a vinyl carbonate, and halogenatedcarbonates.
 3. The electrolyte of claim 2, wherein the halogen isselected from the group consisting of Cl₂, Br₂ and I₂.
 4. Theelectrolyte of claim 2, wherein the oxychloride is selected from thegroup consisting of SO₂Cl₂ and SOCl₂.
 5. The electrolyte of claim 2,wherein the interhalogen is selected from the group consisting of iodinemonochloride (ICl), iodine trichloride (ICl₃) and iodine monobromideI₂Br₂
 6. The electrolyte of claim 1, wherein the oxidizing agentadditive has a stronger oxidizing ability than elemental S.
 7. Theelectrolyte of claim 1, further comprising a dioxolane as a cosolvent.8. The electrolyte of claim 7, wherein the dioxolane co-solventcomprises less than about 20% by weight of the electrolyte solventmixture.
 9. The electrolyte of claim 1, wherein said main solvent is alinear polyether.
 10. The electrolyte of claim 1, wherein said mainsolvent is chosen from the glyme family [ CH₃O(CH₂CH₂O)_(n)CH₃ ]including monoglyme, diglyme, triglyme, and tetraglyme.
 11. Theelectrolyte of claim 1, wherein said main solvent is dimethoxyethane.12. The electrolyte of claim 1, further comprising a second co-solventhaving a donor number of at least about
 13. 13. The electrolyte of claim12, wherein said second co-solvent is at least one ofhexamethylphosphoramide, pyridine, N,N-diethylacetamide,N,N-diethylformamide, dimethylsulfoxide, tetramethylurea,N,N-dimethylacetamide, N,N-dimethylformamide, tributylphosphate,trimethylphosphate N,N,N′,N′-tetraethylsulfamide, tetramethylenediamine,tetramethylpropylenediamine, and pentamethyldiethylenetriamine.
 14. Theelectrolyte of claim 1, further comprising an electrolyte salt.
 15. Theelectrolyte of claim 14, wherein said electrolyte salt is at least oneof LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, LiCF₃SO₃, LiClO₄, LiPF₆, LiBF₄, LiAsF₆.16. The electrolyte of claim 1, wherein said electrolyte is in a liquidstate.
 17. The electrolyte of claim 1, wherein said electrolyte is in agel state.
 18. The electrolyte of claim 1, wherein said electrolyte isin a solid state.
 19. A negative electrode, comprising: a) a lithiummaterial; and b) a film coating said lithium material, said film formedby treating said lithium material with an oxidizing agent prior toincorporation of said electrode into a battery cell.
 20. The negativeelectrode of claim 19, wherein the oxidizing agent is at least one ofsulfur dioxide, nitrous oxide, carbon dioxide, a halogen, aninterhalogen, an oxychloride, a sulfur monochloride, a vinyl carbonateand halogenated carbonates.
 21. The negative electrode of claim 20,wherein the halogen is selected from the group consisting of Cl₂, Br₂and I₂.
 22. The negative electrode of claim 20, wherein the oxychlorideis selected from the group consisting of SO₂Cl₂ and SOCl₂.
 23. Thenegative electrode of claim 20, wherein the interhalogen is selectedfrom the group consisting of iodine monochloride (ICl), iodinetrichloride (ICl₃) and iodine monobromide I₂Br₂.
 24. The electrode ofclaim 19, wherein said lithium material is comprised of at least one oflithium metal, a lithium alloy, and a lithium insertion compound.
 25. Abattery cell comprising: a) a negative lithium electrode; b) a positiveelectrode comprising an electrochemically active material; c) anelectrolyte including a i) a main solvent of an electrolyte solventmixture, having the chemical formula R₁(CH₂CH₂O)_(n)R₂, where n rangesbetween 1 and 10, R₁ and R₂ are different or identical groups selectedfrom the group consisting of alkyl, alkoxy, substituted alkyl, andsubstituted alkoxy groups; and ii) an oxidizing
 26. The battery cell ofclaim 25, wherein the oxidizing agent additive is at least one of sulfurdioxide, nitrous oxide, carbon dioxide, a halogen, an interhalogen, anoxychloride, a sulfur monochloride, a vinyl carbonate, and halogenatedcarbonates.
 27. The battery cell of claim 25, wherein saidelectrochemically active material comprises sulfur in the form of atleast one of elemental sulfur, a metal sulfide, a metal polysulfide. anorganosulfur material, and combinations thereof, wherein said metal isselected from the group consisting of alkali metals, alkaline earthmetals, and mixtures of alkali and alkaline earth metals.
 28. Thebattery cell of claim 25, wherein said electrochemically active materialcomprises Li₂S₈.
 29. The battery cell of claim 25, further comprising afirst dioxolane cosolvent, comprising no more than 20% by weight of theelectrolyte solvent mixture.
 30. The battery cell of claim 25, furthercomprising a second co-solvent having a donor number of at least about13.
 31. The battery cell of claim 30, wherein said second co-solvent isat least one of hexamethylphosphoramide, pyridine, N,N-diethylacetamide,N,N-diethylformamide, dimethylsulfoxide, tetramethylurea,N,N-dimethylacetarnide, N,N-dimethylformamide, tributylphosphate,trimethylphosphate N,N,N′,N′-tetraethylsulfamide, tetramethylenediamine,tetramethylpropylenediamine, and pentamethyldiethylenetriamine.
 32. Thebattery cell of claim 25, further comprising an electrolyte salt. 33.The battery cell of claim 32, wherein said electrolyte salt is at leastone of LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, LiCF₃SO₃, LiC10₄, LiPF₆, LiBF₄,LiAsF₆.
 34. The battery cell of claim 25, wherein said electrolyte is ina liquid state.
 35. The battery cell of claim 25, wherein saidelectrolyte is in a gel state.
 36. The battery cell of claim 25, whereinsaid electrolyte is in a solid state.
 37. The battery cell of claim 25,wherein said electrolyte is a catholyte comprising a dissolved lithiumpolysulfide.
 38. A method of making a protected lithium electrodebattery cell, comprising: a) treating a lithium material with anoxidizing agent to form a negative electrode having a protective film;b) forming a positive electrode comprising an electrochemically activematerial; and c) combining said negative and positive electrodes with anelectrolyte following the treating of said lithium material.
 39. Themethod of claim 38, wherein said positive electrode comprises sulfur inthe form of at least one of elemental sulfur, a metal sulfide, a metalpolysulfide, an organosulfur material, and combinations thereof, whereinsaid metal is selected from the group consisting of alkali metals,alkaline earth metals, and mixtures of alkali and alkaline earth metals.40. The method of claim 38, wherein said electrochemically activematerial comprises Li₂S₈.
 41. The method of claim 38, further comprisinginterposing an electrolyte separator between said positive and negativeelectrodes.
 42. The method of claim 38, wherein the oxidizing agent isat least one of sulfur dioxide, nitrous oxide, carbon dioxide, ahalogen, an interhalogen, an oxychloride, a sulfur monochloride, a vinylcarbonate, and halogenated carbonates.
 43. A method of making aprotected lithium electrode battery cell, comprising: a) forming anegative electrode comprising a lithium material; b) forming a positiveelectrode comprising an electrochemically active material; and c)combining said negative and positive electrodes with an electrolytecontaining an oxidizing agent additive wherein the oxidizing agentadditive reacts with the lithium material of the negative electrode toform a protective film on the negative electrode.
 44. The method ofclaim 43, wherein the oxidizing agent additive is at least one of sulfurdioxide, nitrous oxide, carbon dioxide, a halogen, an interhalogen, anoxychloride, a sulfur monochloride, a vinyl carbonate, and halogenatedcarbonates.
 45. The method of claim 43, wherein the negative electrodeis a glassy coated lithium electrode.
 46. The method of claim 45,wherein a crack in the glassy coated lithium electrode is penetrated bythe oxidizing agent additive contained in the electrolyte.
 47. Themethod of claim 46, wherein the crack is filled with a reaction productbetween the oxidizing agent additive and the lithium material of theglassy coated lithium electrode.